Circadian rhythms are present in species throughout the animal kingdom, and disorders of circadian timing contribute to circadian-based sleep disorders, maladjustment of shift workers and during jet lag, and may contribute to neuropsychiatric disorders including depression and seasonal affective disorder. A transcriptional-translational feedback loop is at the center of the circadian clock mechanism. The phenotype of mice with a dominant-negative mutation in the Clock gene, or of mice with a null mutation in the Bmal1 gene, suggest that these transcription factors are essential for circadian clock function. In this proposal, we will generate mice with conditional disruption of the Clock gene, in which the Clock gene can be disrupted in a tissue specific manner. The work will use molecular, biochemical and behavioral approaches to define the contribution of the Clock gene in the core transcriptional machinery. Furthermore, the tissue-specific gene targeting strategy will for the first time allow us to address important issues regarding the functional importance of oscillators in peripheral tissues. Our hypothesis is that CLOCK is an irreplaceable transcriptional activator, necessary for molecular and physiological rhythmicity. We expect that behavioral and molecular rhythms in brain will be disrupted following a brain-specific disruption of the Clock gene. In mice with a liver-specific disruption of the Clock gene, we expect that brain-based rhythmicity will persist, but local rhythmicity in the liver will be disrupted. These mice will be used to assess the hypothesis that rhythmicity intrinsic to the liver is responsible for rhythmic metabolism of drugs and toxins. The proposed studies will validate a novel method for disruption of circadian function in specific tissues, allowing us to determine the physiological significance of peripheral oscillators. These studies will serve as a "proof of principle" establishing methods for genetic dissection of the circadian oscillator hierarchy and assessment of oscillator function in specific tissues.